I'm not sure what I would give to Gaia as an offering, but use this spell to heal your tree.
Heal a Tree.
Materials needed: Any Candle, a red or white towel, a bowl of clean water.
Step1: Pour the water in a circle around the tree.
Step2: Drape the towel around the roots. Light the candle and put it on the towel.
Step3: Say to the tree;
"Listen to me, I know that someone has gravely hurt thee. Take the warmth, the comfort, and the water, and maybe they stop their slaughter."
Step4: Meditate for a while alongside the tree. Maybe about 30 minutes or longer. Try and stick to any time that uses the number 3. For example, three hours is a good time. Be careful not the burn the tree or the towel.
I hope you can use this and I hope your tree gets better soon. Blessed be!